Vasopressin levels, but not another brain chemical, are linked to daily rhythms in the brain's internal clock of voles

Circadian rhythmic voles exhibit a peak in vasopressin immunoreactive suprachiasmatic nucleus neurons at zeitgeber time 0.

• Daily patterns in neuropeptide levels were observed in rhythmic, weakly rhythmic, and non-rhythmic voles.
• Circadian rhythmic voles showed significant variation in vasoactive intestinal polypeptide and vasopressin neuron numbers.
• Voles with weak or no circadian rhythmicity had similar fluctuations in vasoactive intestinal polypeptide but maintained a continuous high level of vasopressin.
• The suprachiasmatic nucleus in voles does not produce somatostatin or substance P.
• High levels of vasopressin in non-rhythmic voles may indicate impaired release, leading to accumulation in the suprachiasmatic nucleus.
• Vasopressin is associated with the expression of circadian behavior in common voles.

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